Process for the production of carbon molecular sieves

ABSTRACT

A process for producing carbon molecular sieves for the separation of oxygen and nitrogen, whereby finely ground hard coal particles are oxidized with air in a fluidized bed, the coal is then formed after the addition of water and binders and carbonized at temperatures of 800°-900° C., subsequently activated with steam at temperatures of 800°-900° C. and the preliminary product of the low-level activation is treated with carbon splitting hydrocarbons. Starches obtained by reacting agglutinized starches with sulphamates are used as binders.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a National Phase of PCT/EP 91/01796 filed Sep. 20,1991 and based upon German National application P 4 031 580.0 filed Oct.5, 1990 under the International Convention.

FIELD OF THE INVENTION

The invention relates to a process for producing carbon molecularsieves. More particularly, the invention relates to a process of thistype in which finely ground hard coal particles are oxidized with air ina fluidized bed, the coal is shaped after water and binders have beenadded, carbonized, activated with steam and treated with carbonsplitting hydrocarbons.

BACKGROUND OF THE INVENTION

From German Patent 36 18 426 and the therein mentioned German Patent 2119 829 it is known to produce carbon molecular sieves for the separationof gases with small molecular size, particularly O₂ and N₂, in thefollowing manner: finely ground hard coal is oxidized with air in afluidized bed, after adding binders and water the coal is shaped andcarbonized at temperatures up to 900° C., subsequently activated withsteam at temperatures of 800°-900° C. and the preliminary product of thelow-level activation is treated with carbon splitting hydrocarbons at750°-850° C.

The carbon molecular sieves produced this way are used for obtainingnitrogen from air in pressure swins processes. The operating costs ofalternate pressure processes consist primarily of compression costs forthe required air compression. The so-called specific air consumption,i.e. the ratio of the used amounts of air to the produced amount ofnitrogen (m³ air/m³ nitrogen) should be as low as possible. Thisspecific air consumption is directly related to the nitrogenadsorptivity of the carbon molecular sieve, which results from thedifference between the diffusion rates of nitrogen and oxygen in thecarbon molecular sieve.

OBJECT OF THE INVENTION

It is the object of the present invention to produce a carbon molecularsieve with improved oxygen adsorptivity, so that the costs related toenergy consumption can be reduced during the separation of nitrogen fromair.

DESCRIPTION OF THE INVENTION

This object is achieved by using starches as binding agents. Thesestarches can be obtained by reacting agglutinated starches withsulphamates. These starches are described in EP 0 129 227 B1.

The carbon molecular sieves produced according to the invention havegood mechanical strength, comparable to carbon molecular sieves usingcoaltar pitch as binders. The shock resistance (H. von Kienle, E. Bader;"Aktivkohle und ihre industrielle Anwendung"--"Activated Carbon and itsIndustrial Application"--published by F. Enke Verlag, Stuttgart 1980,Page 57) was 90%>0.5 mm., no matter whether coaltar pitch or starch wereused for binders.

The oxygen adsorptivity of the carbon molecular sieve with respect tothe separation of nitrogen and oxygen is determined by a test methodwhich takes into account the differences in the diffusion rates ofnitrogen and oxygen (1-minute test).

An adsorber (capacity 200 ml) filled with carbon molecular sieve to betested is traversed for the duration of one minute by air supplied by anair pump under a pressure of 1013 mbar. During this time the carbonmolecular sieve contained in the adsorber adsorbs gas due to its porousstructure, whereby due to its high diffusion ratio the oxygen ispreferentially adsorbed before the nitrogen. Most of the nitrogen passesthe separating pores and exits the adsorber at the end. After oneminute, this process is interrupted and the adsorber is evacuated incounterflow by means of a vacuum pump to 40 mbar. This way anoxygen-enriched gas is obtained, which is fed to a gasometer. The amountof desorbed considered with respect to the volume of the carbonmolecular sieve CMS (Relative Volume RV=1 Gas/1 CMS), the maximum oxygenconcentration as well as the average oxygen concentration of thedesorption gas are measured. Both oxygen concentrations are higher thanthe oxygen concentration in the air (21% by volume). The resultingrelative volume should range between 2.0 and 2.6. When the relativevolume decreases, the average oxygen concentration increases.

A measure for the oxygen adsorptivity of a carbon molecular sieve is theheight of the average oxygen concentration (at a predetermined RV-value)in the desorption gas of this test. The higher the average oxygenconcentration at the same relative volume, the more advantageous is theseparation efficiency in the technical pressure swins installation. Inthe process of obtaining nitrogen from air, a good carbon molecularsieve has to be capable of adsorbing per time unit the largest possiblespecific amount of oxygen (1 oxygen/1 CMS) during the passage of the airthrough the carbon molecular sieve bulk, so that an oxygen-poor nitrogenleaves the adsorber.

For the comparative evaluation of carbon molecular sieves reference canbe made to the described measured values of the 1-minute test, since theadsorbed oxygen amount can be calculated from the measured values of therelative volume and the average oxygen content. If the 1-minute relativevolume is multiplied by the average oxygen concentration, the resultobtained is the specific oxygen adsorption of the carbon molecularsieve. ##EQU1##

The higher the specific oxygen adsorption (1 O₂ /1 CMS), the better arethe separation characteristics of the carbon molecular sieve.

Within the range of 1 min-relative volumes from 2.0-2.6, the oxygenadsorptivity of the carbon molecular sieve can be evaluated as follows:

    ______________________________________                                        Oxygen Adsorption   CMS Quality                                               ______________________________________                                        <1.05 l/l           normal quality                                            1.05-1.15 l/l       good quality                                              <1.15 l/l           very good quality                                         ______________________________________                                    

The carbon molecular sieves produced according to the invention have aspecific oxygen adsorptivity of >1.15 l/l.

SPECIFIC EXAMPLES Example 1 (Comparative Example)

72 parts by weight bituminous coal with a particle size of 100% smaller80 μm treated with air in the fluidized bed at 230° C. up to an oxygencontent of 12 % by weight are mixed with 28 parts by weight coaltarpitch (soft pitch, softening point according to Kramer-Sarnow at 50°-55°C.) by adding 20 kg water/100 kg of solids (oxidized coaltar bituminouscoal and coaltar pitch) at a temperature of 70° C. The mixture is formedin an extruder into cylindrical bodies having a diameter of 2 mm. Theseformed bodies are carbonized in a continuously running cylindricalrotary kiln, until a final temperature of 880° C. is reached. Thecarbonized product is subsequently activated with steam in a furthercylindrical rotary kiln at a temperature of 810° C. The dwelling time is65 min. The reaction gas contains 75% by volume steam. The activation isconducted so that an activation degree of 5% is achieved. The obtainedactivated goods is subsequently treated in a vibratory oven withbenzene. Thereby a nitrogen-benzene mixture with 8% by volume benzene ispassed in counterflow through the vibratory oven at 800° C. Thecharacteristics of the resulting carbon molecular sieve can be seen inTable 1.

Example 2

66.7 parts by weight of the oxidized hard coal described in Example 1are mixed with 6.7 parts by weight starch (Cerestar AJ 12891 produced byCerestar company n Krefeld) and 26.6 parts by weight water and are thenformed in the extruder into cylindrical bodies with a diameter of 2 mm.After being dried at 105° C., the formed bodies are carbonized andactivated as in Example 1. The benzene treatment was performed in thevibratory oven, as described in Example 1. The very good characteristicsof the carbon molecular sieve can be seen in Table 1.

Example 3

60 parts by weight of the oxidized hard coal described in Example 1 aremixed with 12 parts by weight of starch (Cerestar AJ 12891 produced byCerestar company in Krefeld) and 28 parts by weight water and formed inthe extruder into cylindrical bodies with a diameter of 2 mm. After adrying period at 105° C., the formed bodies are carbonized and activatedas in Example 1. The benzene treatment is performed in a vibratory ovenas described in Example 1. The very good characteristics of the carbonmolecular sieve can be seen in Table 1.

                  TABLE 1                                                         ______________________________________                                        Examples                                                                                  Example 1                                                                     Comparative                                                                   Example  Example 2 Example 3                                      ______________________________________                                        Volume of charge (ml)                                                                       200        200       200                                        Desorbed gas amount                                                                         490        490       466                                        (ml)                                                                          average O.sub.2                                                                             40.8       50.8      52.2                                       concentration                                                                 (% by vol.)                                                                   Relative volume (l/l)                                                                       2.45       2.45      2.33                                       Oxygen adsorption (l/l)                                                                     1.00       1.24      1.22                                       ______________________________________                                    

We claim:
 1. Process for producing carbon molecular sieves for the separation of oxygen and nitrogen, whereby finely ground hard coal particles are oxidized with air in a fluidized bed, then the coal is shaped after water and binders have been added and carbonized at temperatures up to 900° C., subsequently activated with steam at temperatures of 800°-900° C., and the preliminary product of low-level activation is treated with carbon splitting hydrocarbons at 750°-850° C.; characterized in that starch products obtained by reacting agglutinized starches with sulphamates are used as said binders. 